1. Field of the Invention
The present invention relates to a clutch control device installed on, for example, a vehicle for controlling an amount of a stroke of a clutch by employing a motor.
2. Description of the Related Art
As a conventional clutch control method, an actuator is installed in a clutch for dry single-plate starting clutch, and the actuator adjusts, by changing an amount of a stroke of the clutch, an engaging force of the clutch as described in Japanese Patent Application Laid-open No. 2002-81472, for example.
The control method according to Japanese Patent Application Laid-open No. 2002-81472 prevents, by preventing an excessive engagement of the clutch even when a vehicle speed increases, a stop of an engine upon sudden braking, a generation of shocks upon acceleration, and the like.
In this way, it is necessary, for preventing the stop of the engine during creeping at low speed, or upon a slow start, to highly precisely control the engaging force of the clutch. Moreover, for a so-called automatic manual transmission provided with a clutch, it is necessary, for preventing generation of a shift shock during a gear shift, to highly precisely control the engaging force of the clutch after once the clutch is disengaged, and a gear position is changed.
For this purpose, when a motor is used as an actuator for changing the amount of the stroke of the clutch, and a mechanism in which a rotation angle of the motor is proportional to the amount of the stroke of the clutch is provided, in order to control the engaging force of the clutch, it is necessary to highly precisely control torque of the motor. Moreover, the motor torque is proportional to a current supplied to the motor, and thus, in order to increase a precision of the control of the motor torque, it is necessary to increase a precision for detecting the motor current.
As a technology for detecting the motor current, in order to restrain influence of current ripples upon detection of the motor current, there is known a technology for detecting the motor current at a predetermined timing according to the rotation angle of the motor as described in Japanese Patent Application Laid-open No. 2006-6037, for example.
A description is now given of an operation for an automatic transmission using a motor as an actuator for changing an amount of a stroke of a clutch from a state in which the clutch is completely disengaged to a state in which the clutch is engaged.
It should be noted that a clutch mechanism to be described has a configuration in which the rotation angle of the motor is proportional to the amount of the stroke of the clutch, and the engaging force of the clutch can be controlled by the torque of the motor. Moreover, there is also provided a mechanism in which, when a power is not applied to the clutch by the motor, the clutch is returned to a released (disengaged) state, and it is thus necessary for engaging the clutch to always supply the power from the motor to the clutch.
First, when the clutch is completely disengaged, the motor is not driven, and the current does not flow through the motor.
Then, in order to engage the clutch, the amount of the stroke of the clutch is changed. The clutch is in the completely released state, and hence, in order to rotate the motor, a predetermined drive voltage pattern is supplied to a drive circuit for the motor. After the drive voltage pattern is supplied to the drive circuit, the motor starts rotating after a certain delay. It is necessary to manage the torque of the motor during a period until the motor starts rotating, but the motor current is detected in synchronism with a rotation of the motor according to a conventional technology for detecting the motor current, and it is thus impossible to detect the motor current when the motor is stopping or the motor is rotating at such extremely low speed that a control device cannot detect. As a result, in this state, the motor torque cannot be managed, and cannot be controlled highly precisely.
In other words, the motor is constructed by a plurality of coils, and it is thus necessary to switch a current-supply phase of the coil for each predetermined cycle of the rotation angle. The coils contain an inductance component, and when the current-supply phase of the coils is switched, a current value saturates as the time elapses after the start of the current supply. Therefore, the conventional motor current detection technology detects a current value with a small variation by detecting the motor current immediately before the current-supply phase is switched. However, the switching of the current-supply phase occurs only when the motor is rotating, and thus, it is not possible to detect the motor current when the motor is stopping.
Then, in a period in which the motor starts rotating and the amount of the stroke of the motor starts changing, the motor is rotating, and hence, based on the motor current detected according to the rotation angle of the motor, the motor torque is highly precisely controlled.
A period of time required for engaging the clutch is a part of a period of time required for a gear shift operation, and thus, in order to carry out the gear shift operation as quickly as possible, it is desired to reach an amount of the clutch stroke at which the clutch engages as quickly as possible. It is therefore necessary to accelerate the motor up to the possible highest rotation speed. Moreover, when there occurs a rapid change in the motor torque upon the engagement of the clutch, a shift shock may occur, and it is thus necessary to highly precisely control the motor torque even in a deceleration period until the clutch starts engaging.
Then, when the clutch once engages, a change in the amount of the clutch stroke decreases, and it is thus necessary to control the engaging force of the clutch according to an operation state. The motor is not rotating when the clutch is engaged, and hence it is not possible to detect the motor current according to the rotation angle of the motor. As a result, the motor torque cannot be controlled, and thus, the engaging force of the clutch cannot be controlled.
The engagement and disengagement of the clutch is required to frequently repeat the above-mentioned operation as quickly as possible. In other words, the motor which changes the amount of the clutch stroke is also required to carry out the operation of transitioning from a stop state to a high rotation speed state, and then stopping as quickly as possible. Moreover, a change in the motor torque is proportional to a magnitude of the torque transmitted by the clutch, and thus, it is necessary, independently of the rotation state of the motor, to stably control the motor torque.
However, when the motor is stopping or rotating at such extremely low speed that the control device cannot detect, the motor current cannot be detected, and therefore there arises a problem that the motor torque, which is proportional to the motor current, cannot be highly precisely controlled.
As a method for highly precisely controlling the motor torque, there is known a vector control in which the motor current or magnetic flux linkage is vectorized as an instantaneous value, and is, as an instantaneous vector of the torque, made coincident with target torque. When the vector control is employed, independently of the rotation state of the motor, it is possible to highly precisely control the motor torque. However, for the vector control, it is necessary to sample current data at high speed with respect to the period of the change in the rotation angle, resulting in necessity of an expensive device for arithmetic operation.
In order to solve the above-mentioned problems, even in a case where the motor current is sampled at a constant low-speed sampling period, when the motor current with a small variation can be detected, it is conceivable to highly precisely control the motor torque by means of an inexpensive configuration. On this occasion, in a period in which the sampling period of the current data is equal to or more than twice of a switching period of the current-supply phase of the coils, particularly in a period in which the motor is stopping, the motor current is periodically updated, and it is possible to highly precisely detect the motor current when the motor is stopping.
However, in this case, compared with the method in which the motor current is detected immediately before the switching timing of the current-supply phase, the variation in the motor current detected when the motor is rotating is large, and therefore there arises a problem that the precision for detecting the motor current decreases.
As a result, in a conventional clutch control device, when the rotation speed of the motor is equal to or more than a predetermined rotation speed, the motor current is detected according to the switching timing of the current-supply phase of the motor, and when the rotation speed of the motor is less than the predetermined rotation speed, the motor current is detected in each predetermined period.
As a technology for calculating the rotation speed of the motor, there is provided one which calculate the rotation speed of the motor based on a signal provided by a rotation angle sensor of a motor as described in Japanese Patent Application Laid-open No. 2003-264990, for example.
The calculation method according to Japanese Patent Application Laid-open No. 2003-264990, detects an angle signal input at every predetermined rotation angle from a rotation angle sensor, and calculates a value proportional to a reciprocal of a signal interval as the rotation speed of the motor.
However, the conventional clutch control device has the following problems.
In the conventional clutch control device, when the rotation speed of the motor rapidly decreases, the signal interval of the angle signal from the rotation angle sensor increases, and it is not possible to update the present rotation speed of the motor until the next angle signal is input. Moreover, the rotation speed of the motor is proportional to the reciprocal of the signal interval, and hence an infinitely long period of time is necessary for determining that the rotation of the motor is stopping.
As a result, when the rotation of the motor is stops or the rotation speed of the motor rapidly decreases, and even when an actual rotation speed of the motor decreases below the above-mentioned predetermined rotation speed for switching the detection method of the motor current, it is not possible to update the calculated rotation speed of the motor, and it is not possible to switch the motor current.
Therefore, there arises a problem that, when the rotation of the motor stops or the rotation speed of the motor rapidly decreases, the motor current cannot be highly precisely detected, and thus, the motor torque cannot be highly precisely controlled.